Secretion of relaxin by the cynomolgus monkey has been studied during the menstrual cycle and during pregnancy. In vivo, relaxin was found in peripheral and left and right ovarian vein blood during the luteal phase of the menstrual cycle. Relaxin was found in blood of pregnant monkeys from 40 days gestation to term. Peripheral, left and right utero-ovarian and uterine vein blood contained relaxin. In vitro luteal cells taken from the corpus luteum of the menstrual cycle secreted relaxin in short term cultures (4-48 hrs). Luteal cells from the corpus luteum of early pregnancy and at term delivery secreted relaxin in long term cultures, up to 12 days, with a medium changes each second day.